I've read up on that on this site. I'll have to add that most all of my slabs have some "frosting". I have no doubt that those cards are authentic due to the low grade and condition. They're almost all SGC holders where that is noticed.

Is there a certain amount of frosting that would be easy to determine the slab had def been opened?

There's a difference between frosting and chips or superficial marks. It's hard to explain though. Maybe try buying a low-end nominal card that's slabbed and open it. See what it looks like when you put it back together.